Conventional clamping device for screws usually includes a clamping part, the clamping part having a handle for receiving pneumatic or electric tools, a shaft body with a space inside for inserting a tool shaft. The shaft body having a sliding sleeve at its front end for pressing a steel ball against a groove disposed on the middle part of the tool shaft, the sliding sleeve is pressed against by a spring to keep it in position, can slide the spring to let the steel ball to have room to move backwards, so that the tool shaft can be released to disconnect from the clamping device. The tool shaft having dual-connecting ends with its one end having a receiving hole for connecting a tool head, another end having an outer threading, a nut is sleeved on the outer threading, an inner threading of an outer sleeve is also sleeved on the outer threading. The front end of the outer threading of the tool shaft having a hole for the tool head, the front end of the outer threading inside the outer sleeve having an elastic element and a screw attraction element. The screw attraction element having a magnet and a pad. Accordingly, the outer sleeve can adjust the depth of the locking mechanism, and can press against the nut for positioning purpose, for clamping different tools. The structure is quite complicated. Besides, it cannot fix the tool head and also position the screw and adjust the depth of locking at the same time.